1. Technical Field
This invention relates generally to densitometers for measuring the relative optical density of a developed toner image on a test patch for controlling process parameters in electrostatographic apparatus such as copiers and/or printers.
2. Background Art
In electrostatographic apparatus such as printers and/or copiers, regulation of process control parameters is required to produce images having constant and predeterminable densities. Such control parameters include electrical charger energization, exposure energy, development bias voltage, toner concentration, and image transfer potential.
One method of monitoring the effect of the process control parameters is to measure the transmissive or reflective optical density of a toner image on an exposed and developed area (called a "patch") of an image receiver. Known techniques for measuring the optical density of the developed toner image include the use of infrared densitometers.
According to a typical prior art device, such densitometers generally include a light generator such as an LED., a transducer for converting the light transmitted by, or reflected from, the patch to an electrical signal current; and a sensing circuit for comparing the signal current to a reference current, and for producing a difference or ratio signal. Such densitometers suffer from a dependency on the provision of a constant current, both to the light generator and for the reference current; as fluctuations in one of these currents would affect the difference or ratio signal. Accordingly, densitometers are usually powered by at least one constant current source so that fluctuations in line voltage do not affect either the amount of current to the light source or the reference current.
In other prior systems, at least one of the constant current sources is replaced by a constant intensity illumination source. See for example U.S. Pat. No. 4,553,033, which issued to Hubble et al. on Nov. 12, 1985.